The invention relates to thermostructural composite material bowls. A field of application of the invention is more particularly manufacturing bowls for receiving crucibles containing molten metal, such as silicon.
The term xe2x80x9cthermostructural composite materialxe2x80x9d is used to mean a material comprising fiber reinforcement made of refractory fibers, e.g. carbon fibers or ceramic fibers, and densified by a refractory matrix, e.g. of carbon or of ceramics. Carbon/carbon (C/C) composite materials and ceramic matrix composite (CMC) materials are examples of thermostructural composite materials.
A well-known method of producing silicon single crystals in particular for manufacturing semiconductor products consists in melting silicon in a crucible, in putting a crystal germ having a desired crystal arrangement into contact with the bath of liquid silicon, so as to initiate solidification of the silicon contained in the crucible with the desired crystal arrangement, and in mechanically withdrawing an ingot of single crystal silicon obtained in this way from the crucible. This method is known as the Czochralski method or as the xe2x80x9cCZxe2x80x9d method.
The crucible containing the molten silicon is usually made of quartz (SiO2). The crucible is placed in a bowl which is generally made of graphite, it being understood that proposals have also been made to make the bowl at least in part out of C/C composite materials. The bottom of the bowl stands on a support. For this purpose, the bottom of the bowl must be machined, in particular so as to form a bearing surface for centering purposes and also a support zone. In addition, in the application in question, very high purity requirements make it necessary to use raw materials that are pure, with methods that do not pollute them, and with methods of purification in the finished state or in an intermediate state of bowl manufacture. For carbon-containing materials (such as graphite or C/C composites), methods of purification by high temperature treatment (at more than 2000xc2x0 C.) under an atmosphere that is inert or reactive (e.g. a halogen) are known and are commonly used.
The pieces of graphite used as bowls are fragile. They are often made up of as a plurality of portions (so-called xe2x80x9cpetalxe2x80x9d architecture) and they cannot retain molten silicon in the event of the crucible leaking. This safety problem becomes more critical with the increasing size of the silicon ingots that are drawn, and thus with the increasing mass of the liquid silicon. Furthermore, graphite bowls are generally of short lifetime while being thick and thus bulky. It is preferable to use C/C composite material pieces which do not present those drawbacks and which, in particular, present better mechanical properties.
The manufacture of a C/C composite material piece, or more generally a piece of thermostructural composite material, usually comprises making a fiber preform having the same shape as the piece that is to be made, and that constitutes the fiber reinforcement of the composite material, and then densifying the preform with the matrix.
Techniques presently in use for making preforms include winding yarns by coiling yarns on a mandrel having a shape that corresponds to the shape of the preform that is to be made, draping which consists in superposing layers or plies of two-dimensional fiber fabric on a former matching the shape of the preform to be made, the superposed plies optionally being bonded together by needling or by stitching, or indeed by three-dimensional weaving or knitting.
The preform can be densified in well-known manner using a liquid process or a gas process. Liquid process densification consists in impregnating the preformxe2x80x94or in pre-impregnating the yarns or plies making it upxe2x80x94with a matrix precursor, e.g. a carbon or ceramic precursor resin, and in transforming the precursor by heat treatment. Gas densification, known as chemical vapor infiltration, consists in placing the preform in an enclosure and in admitting a matrix-precursor gas into the enclosure. Conditions, in particular temperature and pressure conditions, are adjusted so as to enable the gas to diffuse into the core of the pores of the preform, so that on coming into contact with the fibers it forms a deposit of matrix-constituting material thereon by one of the components of the gas decomposing or by a reaction taking place between a plurality of components of the gas.
For pieces that are of relatively complex shape, for example bowl shaped, a particular difficulty lies in making a fiber preform having the right shape.
Another difficulty lies in obtaining densification in a manner that is reasonably simple and fast, in particular for bowls of large dimensions. Unfortunately, in the semiconductor industry, there exists a need for silicon ingots of ever greater diameter, thus requiring crucibles and support bowls to be provided that are of corresponding dimensions.
An object of the invention is to propose a method of manufacturing a bowl of thermostructural composite material that makes it possible to overcome the above difficulties, while remaining simple and low in cost.
According to the invention, the method comprises the steps which consist in:
making a bowl preform by winding a yarn, the preform having an axial passage through its bottom;
densifying the bowl preform by chemical vapor infiltration; and
closing the passage by means of a plug.
Making a bowl preform with an axial passage presents two advantages. Firstly, the preform can be made by winding yarn without special difficulty. This would not be the case if a complete bowl preform had to be obtained by winding the yarn. In addition, while the preform is being densified by chemical vapor infiltration, the presence of an axial hole enhances flow of the gas and thus enhances densification.
A stiffened or consolidated bowl preform is preferably obtained prior to performing densification by chemical vapor infiltration. In conventional manner, a consolidated bowl preform is made by partial densification of a fiber structure having the desired shape, with the partial densification being at least sufficient to enable the preform to be handled. Partial densification can be performed by a gas process, or it can be performed by a liquid process, using impregnation by means of a precursor of the material that constitutes the matrix of the composite material, and transforming the precursor by heat treatment.
The perform can be consolidated by impregnation with a carbon precursor, e.g. selected from phenolic, furan, epoxy, and polyimide resins, and then transforming the precursor.
A consolidated preform is advantageously made by winding a yarn impregnated with said precursor.
Two consolidated preforms can be made simultaneously on a mandrel of a shape that corresponds to that of two facing bowl portions, with the yarn being wound over the mandrel and with the resulting winding being cut in its middle portion.
Densifying the preform by chemical vapor infiltration makes it possible to obtain a carbon matrix having the continuity necessary to ensure that the installation for producing a silicon single crystal is not polluted with particles that come from the fibers or from resin coke formed on the fibers in order to consolidate the preform. A carbon matrix obtained by chemical vapor infiltration also presents better ability to withstand corrosion on coming into contact with a quartz crucible at high temperature.
Advantageously, the consolidated bowl preform is made from yarn that has no surface treatment, e.g. oxidation under controlled conditions using electrochemical or other means. In particular, the yarn can be a carbon yarn. The absence of the surface treatment which is usually provided on commercially-available yarns for providing surface functions that encourage bonding with organic matrices contributes to obtaining better dimensional stability by avoiding the appearance of internal stresses while making the composite material.
In another feature of the method, the method includes a step which consists in performing a final chemical vapor infiltration step after the passage has been closed by the plug, with the plug itself preferably being made of thermostructural composite material. The final infiltration step can include forming a matrix of a kind that is different from that formed previously during the steps of consolidating the bowl preform and the subsequent densification thereof. Thus, with a carbon densified preform, the final infiltration step can consist in depositing a ceramic matrix, e.g. of silicon carbide. Such an outer matrix material provides the composite material with protection against oxidation.
Advantageously, the bowl is subjected to high temperature purification and stabilization treatment, preferably at a temperature greater than 2200xc2x0 C. Purification can be performed under an atmosphere of chlorine, as is well known for graphite. It makes it possible to remove impurities that might pollute the silicon when using the bowl as a support for crucibles containing silicon for manufacturing single crystal silicon ingots.
Such purification treatment can be performed on the consolidated bowl preform. The heat treatment then also contributes to avoiding dimensional variations during the subsequent manufacturing processes. Providing the plug closing the bottom of the bowl has also been subjected to purification treatment, performing purification after chemical vapor infiltration need not be necessary.
A protective coating can be formed at least on the inside of the bowl. Such a coating can be of pyrolytic carbon or xe2x80x9cpyrocarbonxe2x80x9d, obtained by chemical vapor deposition, or it can be of ceramic, e.g. silicon carbide (SiC) likewise obtained by chemical vapor infiltration. In a variant, the inside face of the bowl can be provided with a protective layer, e.g. made of C/C composite material.